1. Field of the Invention
This invention relates generally to optics in which a single channel is changed to plural channels and more particularly to a method and apparatus for splitting a beam of energy into two diverging beams of energy without interference from secondary images.
2. Description of the Prior Art
Energy beam splitters, and in particular optical energy beam splitters, are extremely useful in industries where a beam of energy is split into two beams of energy and each resultant beam is used for a particular function.
One such use of beam splitters is in optical communication systems which utilize a transmitter and a remote receiver. The transmitter utilizes a LASER to produce an optical LASER beam carrier on which modulated information is placed. An optical beam splitter is placed in the path of the optical carrier to split the optical carrier into two primary beams. One primary beam is modulated with the information and travels to the receiver. The other primary beam is used in a feedback circuit to keep the frequency and amplitude of the LASER beam carrier constant. The receiver uses an optical beam combiner to split an optical signal produced by a local oscillator and impinging on the optical beam combiner into two beams of energy and to split the incoming modulated optical carrier impinging thereon into two beams of energy and then combining the resultant beams of energy into a pair of combined beams. One of the combined beams is detected by an optical detector in the receiver. This first combined beam improves the sensitivity of the detector. The second combined beam is fed back to the local oscillator for controlling the same. Beam splitting devices are also useful in the field of holography and in optical instruments such as interferometers where two beams are used to produce interference patterns.
Typically, energy beam splitters consist of a plane parallel or substantially plane parallel piece of polished transparent material having a first and second surface. An incoming beam of energy is picked up by the first surface and the incoming beam of energy is split by the first surface into an externally reflected beam and an internally transmitted beam. The transmitted beam is split by the second surface into an externally transmitted beam and an internally reflected beam. The externally reflected and transmitted beams are the two primary useful beams.
Although the typical plane parallel beam splitter splits the incoming beam into two resultant energy beams, it does have a distinct disadvantage in that when the internally reflected beam is reflected back to the first surface it is split into a second externally transmitted beam and a second internally reflected beam. The second internally reflected beam is split into a third externally transmitted beam and a third internally reflected beam at the second surface. These multiple internal reflections which result in multiple externally transmitted and reflected beams continue throughout the energy beam splitter. The multiple externally transmitted beams interfere with the primary externally reflected and transmitted beams causing detrimental effects. These detrimental effects are that each of the secondary transmitted and reflected beams will combine with the primary transmitted and reflected beams causing a change in intensity of the primary transmitted and reflected beams. It then becomes extremely difficult to properly predict the intensities of the externally reflected and transmitted beams because they are dependent on precise knowledge of the thickness of the beam splitter, the index of refraction, and the angle of incidence of the incoming beam.
In the past, anti-reflective coatings have been applied to the second surface to eliminate the internally reflected beam, however, due to the imperfections in the coatings they have not been successful in completely eliminating the partial external transmissions of the internal reflections. Another device used is a slightly wedge shaped energy beam splitter which eliminates direct interference by the secondary transmitted and reflected beams by separating them from the primary externally reflected and transmitted beams, however, the energy beam splitter still produces the unwanted secondary externally transmitted and reflected beams which are substantially collinear with the primary externally reflected and transmitted beams and therefore are detrimental to the detection of the primary externally reflected and transmitted beams.
When the energy beam splitting device is used as a beam combiner the same detrimental effects are inherent therein as previously described for the typical energy beam splitting device.